In the figure, the chord AB of the circle with centre O, is produced to C such that BC = OB. CO is joined and produced to meet the circle in D. If ∠ACD = y and ∠AOD = x, show that x = 3y.
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Solution
In △ OBC, OB = BC
⇒ ∠BOC = ∠BCO = y (angles opposite to equal sides are equal)
∠OBA is the exterior angle of ∆BOC
So, ∠ABO = 2y (exterior angle is equal to the sum of interior opposite angles]
Similarly, ∠AOD is the exterior angle of ∆AOC
∴ x = 2y + y = 3y.